1. Field of the Invention
The present invention relates to a radio communication system, and more particularly, to a realization of a multiplicity of fixed stations having small spatial radio zones for radio connection of mobile stations.
2. Description of the Related Art
As one of the prior art radio communication systems of the type referred to, there is a portable radio telephone system of a cellular type which arrangement is schematically shown in FIG. 18.
In FIG. 18, a portable telephone control station 201 is connected at its one side to a wired telephone network 202 and also connected at the other side to radio base stations 203-1 to 203-3 as master stations through respective wired lines 206-1 to 206-3. These radio base stations 203-1 to 203-3 have respective spatial radio zones 204-1 to 204-3. When a portable radio telephone set 205 as a mobile station is located within the spatial radio zone 204-2, the portable radio telephone set 205 can communicate with the radio base station 203-2 through the spatial radio circuit and further can communicate with the wired telephone network 202 through the portable telephone control station 201.
If the portable radio telephone set 205 issues a calling signal, then the calling signal issued from the portable radio telephone set 205 is received at any of the respective radio base stations 203-1 to 203-3 even when the portable radio telephone set 205 is located within any of the spatial radio zones 204-1 to 204-3, so that interconnection control between the portable radio telephone set 205 and the wired telephone network 202 is carried out and a communication is started.
FIG. 19 is a block diagram of an internal arrangement of the portable radio telephone set 205, and FIG. 20 is a sequential chart for explaining the brief operation of the portable radio telephone system when a calling signal is issued from the portable radio telephone set.
Explanation will be made as to the case where a calling signal is issued from the portable radio telephone set 205.
First of all, when a hook switch (not shown) of the portable radio telephone set 205 is turned ON, a controller 259 of the portable radio telephone set 205 detects the off-hook state, sets a synthesizer 258 at a frequency of a control channel, and applies to a transmitter 252 a calling signal A including an identification number of the portable radio telephone set 205. And a modulation output of the transmitter 252 is amplified at a power amplifier 253 and then applied through an antenna duplexer 254 to an antenna 255, whereby the calling signal A is transmitted from the antenna.
The radio base station 203-2, when receiving the calling signal A from the portable radio telephone set 205, transmits a calling signal B including identification numbers of the radio base station 203-2 and the portable radio telephone set 205 through the wired line 206-2 to the portable telephone control station 201.
The portable telephone control station 201, when determining that the received identification numbers are for use in its own portable radio telephone system, returns to the radio base station 203-2 a calling answer signal C including the designation of a speech channel while the control station 201 transmits a calling signal D to the wired telephone network 202.
The radio base station 203-2, when receiving the calling answer signal C, transmits a calling answer signal E to the portable radio telephone set 205 through the control channel.
In the portable radio telephone set 205, an input received at the antenna 255 is demodulated at a receiver 256 to form the calling answer signal E which is then applied to the controller 259. This causes a display (not shown) of the portable radio telephone set 205 to indicate thereon a message telling the user of the portable radio telephone set 205 that the user can dial.
Under such a condition, when the user of the portable radio telephone set 205 depresses a dial key (not shown) mounted thereon to enter a desired dial number, this causes the controller 259 to apply a dialing signal F indicative of the dial number to the transmitter 252 to transmit the dialing signal F from the antenna 255.
The radio base station 203-2, when receiving the dialing signal F, transmits a dialing signal G indicative of the dial number to the wired telephone network 202 through the wired line 206-2 and the portable telephone control station 201. As a result, a party telephone set as speech party corresponding to the dial number is called so that, when the speech party answers the calling, the speech can be started. A down voice signal from the wired telephone network 202 is transmitted through the portable telephone control station 201, radio base station 203-2 and a spatial down voice circuit to the portable radio telephone set 205. On the other hand, an up voice signal transmitted from the portable radio telephone set 205 is transmitted through a spatial up voice circuit, the radio base station 203-2 and the portable telephone control station 201 to the wired telephone network 202.
In such a portable radio telephone system, the major problem is the capacity for accommodating the portable radio telephone sets within radio zones. In other words, with the prior art cellular type portable radio telephone system, since the transmission output of the portable radio telephone set is as large as several watts, it has been necessary to set a large distance among each of the radio base stations in order to avoid any interference between the portable radio telephone set and the radio base station using the same frequency. Thus, the number of portable radio telephone sets to be accommodated within the unit area per the entire frequency band allocated to the portable radio telephone system must inevitably be small. This has led disadvantageously to a poor frequency use efficiency.
Two methods for improving the capacity for accommodating portable radio telephone sets have been recently proposed. One method is to set small the transmission outputs of the radio base station and the portable radio telephone set; while the other method is to digitize a voice signal to employ a time division multiple access (TDMA) communication system. When the transmission outputs of the radio base station and portable radio telephone set are set small, that is, when it is desired to secure the same service area as the portable radio telephone system having a large spatial radio zone, this requires the number of radio base stations to be increased, involving an economical problem. Further, unless such a large number of radio base stations are not installed, there occurs such a problem that an unserviceable area is created. In addition, when the number of radio base stations is thoughtlessly increased to solve the aforementioned problems, this also involves other problems such as a scene appearance problem, locations for installation of the system, and in particular, the necessity of an AC power source.
When a voice signal is digitized to employ the time division multiplex access communication system, the frequency band is divided into a predetermined number of time slots as shown in FIG. 21, for example, and speech channels and the time slots are managed in a centralized manner by the portable telephone control station as previously explained above.
In the example of FIG. 21, one frame is divided into 6 time slots (SLOT 1 to SLOT 6) to be repetitively used. Since the frequency band per radio channel can be made to be the same, a frequency effective utilization can be 6 times increased. In other words, 6 times the number of portable radio telephone sets used in the prior art can be used.
In this case, the aforementioned calling answer signals C and E each contains the speech channel number and the number of any one of the 6 time slots and the speech is carried out with use of the designated time slot in the designated speech channel.
Meanwhile, the portable radio telephone system based on the time division multiplex access communication system can digitize a voice signal and can increase relatively easily the number of portable radio telephone sets to be used, but has a limit of increasing the number of portable radio telephone sets. That is, when the number of divided time slots is increased, this requires high-speed data transmission and thus a wide frequency band, which results in that the frequency use efficiency cannot be increased.
On the other hand, as the transmission output or power is made smaller, the capacity for accommodating portable radio telephone sets increases, the consumption current of the portable radio telephone set also decreases, its battery can last longer, and thus the operating time of the portable radio telephone set can be made longer. However, it is difficult to install an increased number of radio base stations efficiently and economically.